This invention relates to an optical information recording medium capable of recording, reproducing and rewriting information signals at high density and high speed by using optical means such as a laser beam.
The technology of recording and reproducing information at high density by using a laser beam is known, and it is presently applied in document filling system, still picture filing system, and others. Rewritable type information recording systems are also researched and developed, and actual cases have been reported. The active layer responsible for recording in optical disk is mainly composed of chalcogen such as Se and Te or its compound (chalcogenide). In these substances, an amorphous phase is obtained relatively easily by heating and cooling, and the optical constant varies between the crystal phase and amorphous phase. By optically detecting this phenomenon, the information is reproduced.
The amorphous phase is obtained, for example, by emitting strong and short pulse laser light to heat the illuminated part up to the liquid phase, and then quenching. On the other hand, the crystal phase is obtained, for example, by emitting weak and long pulse laser light, and heating and gradually cooling the amorphous phase. The change in the optical constant is mainly observed as a change in the reflectivity.
In the case of rewritable type optical disk device, generally, the amorphous phase corresponds to the recording signal, and the crystal phase corresponds to the erased state. That is, the state of the amorphous recording marks arranged on the track in crystal state is the recorded pattern of the information.
As practical conventional examples of recording film material, Te and Ge were used as main materials. This is because Te is stable as a crystal at room temperature, but does not exist in an amorphous state, and Ge is added as an additive in order to stabilize the amorphous state at room temperature.
As optical recording material using mainly Te-Ge, for example, Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2 (Appl. Phys. Letters, 18, 254, 1971) is known. In this composition, however, the crystallization (erasure) time is long, about scores of microseconds, and the contrast ratio of recording pattern is not sufficient, and hence it was not satisfactory practically.
Besides, a Te-Ge-Sn-Au alloy (U.S. patent application Ser. No. 890,325) and a Te-Ge-Sb-Se alloy (U.S. patent application Ser. No. No. 909,673) have been also reported. In these compositions, the insufficient characteristics in the Te-Ge compound have been improved. This is considered because Au or Sb works by promote crystallization by partially destroying the strong covalent structure, and the crystallization time is less than several microseconds.
Since the phase change from amorphous to crystal takes a longer time than the phase change from crystal to amorphous generally, shortening of the crystallization time is the key for improving the transfer rate of a signal.
Although recording film materials of which crystallization time is less than several microseconds have been proposed, it is needed to further shorten the crystallization time in order to enhance the performance of optical disk. That is, in the recording film materials proposed so far, the crystallization speed may not be always sufficient, and it may be impossible to apply to higher transfer rate of signals in the future.